Fault-current circuit breaker are used in electrical systems, in order to protect personnel against dangerous body currents. When a fault current occurs, the circuit breaker disconnects the conductors of a conductor network. The circuit breaker is used as an autonomous unit, or else as an additional module for a switching device. Such an additional module is referred to as a circuit breaker accessory.
With regard to circuit breakers, a distinction is drawn between main voltage independent, so-called FI switches (fault-current circuit breakers) and main-voltage-dependent DI circuit breakers (differential-current circuit breakers). Both switch types have a core-balance current transformer, through which the conductors of a conductor network are passed. A control winding is wound around the core-balance current transformer and is connected to an evaluation unit, via which a release is actuated. When an unacceptable fault current occurs in the conductor network, this is detected by the core-balance current transformer with the associated evaluation unit, and the release disconnects the conductors of the conductor network via a switching mechanism. The fault current at which the circuit breaker responds is referred to as the tripping fault current. The ratio of the tripping fault current to the so-called rated fault current is fixed and is defined by Standards for the various fault current types. The rated fault current is a measure of the protection class for which the respective circuit breaker is designed.
As a rule, FI/DI circuit breakers have a test device, by which the reliability of the circuit breaker can be checked. Such a test device normally connects two primary conductors to one another via a series circuit comprising a test resistor and a test winding and via a test contact (pushbutton) which can be operated, forming a test circuit. A fault current is thus produced when the test contact is closed, and this is detected by the core-balance current transformer together with the associated evaluation unit. A test device such as this can be found, for example, in the article “Fehlerstrom-Schutzschalter zum Schutz gegen gefährliche Körperströme”, [Fault current circuit breaker for protection against dangerous body currents] etz, Volume 110 (1989), Issue 12, pages 580–584.
When two conductors are short-circuited for test purposes, there is a problem in that, in some circumstances, a test current will flow permanently via the test circuit for as long as the test contact is operated. This problem occurs when connection of the test circuit to the conductors of the conductor network is made on the mains voltage supply side, that is to say upstream of the circuit breaker switching mechanism, so that current continues to flow in the test circuit even after the conductors have been disconnected by the circuit breaker during the test. Thus, in conventional test devices, an auxiliary switch which is coupled to the switching mechanism of the circuit breaker is often connected in the test circuit, and interrupts the test circuit when the circuit breaker trips, in order to ensure that the current flow is reliably interrupted. However, the arrangement of an auxiliary switch on the one hand requires additional measures and on the other hand is not always possible, for example in the case of FI/DI accessories, since there is no switching mechanism for space reasons. If it is impossible to arrange any auxiliary switches, the test circuit must therefore be designed, for example, for permanently flowing test current. The design for permanent excitation is extremely complex, in particular when the circuit breaker is designed for high-rated fault currents.